Various types of semiconductor devices are manufactured in much the same way. A starting substrate, usually a thin wafer of silicon or gallium arsenide, is masked, etched, and doped through several process steps, the steps depending on the type of devices being manufactured. This process yields a number of die on each wafer produced. The die are separated with a die saw, and then packaged into individual components.
During the packaging process, several semiconductor die are attached to a lead frame, often with a material such as metal, or an epoxy or other viscous adhesives. Bond wires couple each of several bond pads on each die to conductive leads on the lead frame. The die, the wires, and a portion of the leads are encapsulated in plastic or encased in ceramic. These leads couple the die with the device into which the component is installed, thereby forming a means of I/O between the die and the device.
One step of semiconductor manufacture that is not without problems is the die-lead frame attachment. During the manufacturing process, several die are attached to the lead frame, bond wires are connected from the bond pads on each of the die to the "fingers" on the lead frame, then the die is encapsulated in a protective plastic casing. The plastic packages are separated, and the leads are formed into a desired shape. The lead frame, part of which will eventually form the conductive leads of the components, contains a major surface to which the die is attached, called the "paddle." The die is normally bonded to the paddle with epoxy or metal, although thermoplastic, tape, or another materials are also used. FIG. 1A is a top view, and FIG. 1B is a cross section, showing a conventional lead frame 10 having die paddles 12 with die 14 attached. The frame 10 comprises dam bars 16 which restrict the flow of encapsulating material during encapsulation, exterior leads 18 which are unencapsulated, and lead fingers 20 which will be encapsulated. Bond wires 22 electrically couple bond pads 24 on the die 14 with the lead fingers 20. The adhesive 26 used to attach the die 14 to the lead frame 10 is dispensed on the die paddle area 12 of the lead frame 10. The die 14 is placed on the uncured epoxy (for example) and held at a specific pressure by die attachment equipment having a surface contact tool or an edge contact only tool (collet). The die is pressed down into the adhesive at a specific pressure, and often at a controlled temperature, by the tool and held in place long enough to ensure adhesion. X-Y movement (scrub) is sometimes used to increase adhesion and to speed the process. The attach process often requires a follow-on cure in a separate cure oven. After the attach process, the assembly within the dam bars 16 is encapsulated. The paddle 12 of the lead frame 10 is usually at a lower plane 28, which allows better control of the plastic encapsulation material as it is being injected into the mold. This lessens the chance, for example, of the bond wires 22 detaching from the lead fingers or bond pads.
Various problems are associated with the connection of the die to the die paddle. Occasionally a corner of the die will crack, thereby making the semiconductor inoperable. This can result from stress placed upon the die by the adhesive due to an uneven thermal coefficient of expansion between the die and the adhesive. After the die is attached to the lead frame and oven cured, the assembly is heated at the wire bond step to attach the wire to the die pad. If the die and the adhesive expand at different rates, undue stresses can be inflicted on the die. Cracks can also occur from stress on the die due to shrinkage of the adhesive as it cures, although in recent years chemical improvements in adhesive has reduced this cause of cracking.
Occasionally the die and epoxy may come loose from the lead frame, a problem referred to as "popping die." Popping die can result from too little adhesive under the die, a poor bond between the adhesive and the paddle, or from bowing of the die paddle from heat or pressure. This can be a serious problem, not only because it results in scrapping the die but also because the loose die can damage the molds which are used to encapsulate the package.
Other problems are also associated with the lead frame. Once the die is attached to the die paddle, bond wires electrically couple bond pads on the die to lead fingers on the lead frame. Before encapsulation the lead fingers can move, thereby damaging the bond wire coupling the bond pads and lead fingers which results in an unreliable or nonfunctional device.
Another problem associated with packaging of a semiconductor device is that a new lead frame design must be used each time a die size changes. With conventional lead frames, the die paddle must be approximately the same size as the die, and therefore as the die decreases in size a lead frame with a smaller paddle must be used. Also, the lead fingers must be sufficiently close to the die to keep the bond wires which couple the lead fingers to the bond pad as short as possible. Long bond wires require more material (usually gold) than shorter wires and are therefore more expensive, and also have a greater likelihood of shorting or breaking. When a die is shrunk a new lead frame having a smaller die paddle and longer lead fingers must be produced. A die shrink therefore requires a retooling of the lead frame stamp, which currently costs more than $50,000. In addition, unused stock is discarded or recycled.
U.S. Pat. No. 4,868,635 to Frechette et al. describes a lead frame which can be customized to accommodate semiconductor dies of different sizes. This lead frame, however, would have the problem of lead movement, as can be determined from studying FIG. 3 of Frenchette et al.
A lead frame design which solved the problems associated with the die paddle and long leads, and those resulting from a die shrink would allow for reduced costs and increased yields.